Old school Blaupunkt 8-track player makes whirring sound...

I'm trying to keep my Porsche 914 as original as possible, so I just bought a mid-70s Blaupunkt CR-4081 head unit (am/fm/8 track). I hooked it all up to my 110v to 12v converter and everything worked great, FM came in nice and 8-track sounded like the music was being played in a tin can but otherwise worked great. I take it out to the car and hook everything up. FM works crystal clear, but when I put an 8-track in, it makes this whirring sound with no music. Maybe grounds? don't really know a whole lot about car audio. Was wondering if anyone else has seen this problem maybe with a cassette deck or that one other person out there who is still trying to rock the 8-tracks...